A conventional chip resistor of this kind includes an insulating substrate, a pair of main electrodes formed on a surface of the insulating substrate and spaced from each other, a resistor film which is formed on the surface of the insulating substrate to bridge between the main electrodes and provided with a trimming groove for resistor adjustment, a protective coating formed to cover the resistor film, and a metal plating formed in electrical conduction with each of the main electrodes, as disclosed in Japanese Patent Application Laid-open Nos. 56-148804 and 4-102302 for example. The protective coating includes an undercoat layer of glass formed on the resistor film, a middle-coat layer of glass formed on the undercoat layer and an overcoat layer formed on the middle-coat layer. The trimming groove may be formed by irradiating the resistor film with e.g. a laser beam from above the undercoat layer. The metal plating is formed by solder-plating treatment after performing nickel-plating for the chip resistor as a whole.
The purpose of forming the middle-coat layer of glass on the chip resistor having the above arrangement is to seal the resistor film hermetically against the exterior air with the use of the glass-made middle-coat layer, and to fill up the trimming groove of the resistor film. However, it has been found that if the middle-coat layer is not properly formed, pinholes tend to be formed in the trimming groove, thereby breaking the sealing condition as described below.
Specifically, the middle-coat layer is formed by applying a glass paste by screen-printing and baking it at a substantially high temperature. However, at the time of applying a glass paste by screen-printing for the middle-coat layer, the glass paste often traps bubbles in the trimming groove formed in the resistor film and the undercoat layer. The bubbles will expand upon baking the glass paste, leading to generation of pinholes communicating with the exterior air. As a result, an amount of plating liquid flows into the pinholes during metal plating treatment performed subsequently, and metal plating pieces will grow therein.
If the pinholes are small, the growth of the metal plating pieces within the pinholes does not cause any particular problem. However, if the pinholes are large, the grown metal plating pieces in the pinholes will bridge between the side surfaces at the trimming groove, thereby causing the overall resistance of the resistor film to deviate from a predetermined value. Thus, conventionally, there has been a problem of high occurrence of rejective devices.